For decades, paleontologists have debated the identity of the first animal to inhabit Earth. A recent international study indicates that early representatives of the animal kingdom may have been simple and primitive: sea sponges. Contrary to the prehistoric monsters often depicted in science fiction, these organisms lacked organs, a nervous system, and a mineralized skeleton. Nevertheless, they could serve as a link between the most basic forms of life and the tremendous biodiversity that emerged during the Cambrian period.
This discovery is grounded not in traditional fossils but in “chemical fossils”: molecular remnants that have been preserved in rocks for hundreds of millions of years. Researchers examined a group of molecules known as steranes, which are derived from sterols found in the cell membranes of eukaryotes. These steranes are unique to certain modern sponges, particularly demosponges.
These molecules were detected in Ediacaran rocks, which predate the Cambrian period by over 541 million years, and are also produced by living sponges today. To verify their biological origin, scientists reproduced the synthesis of sterols in a laboratory setting using sponge genes, confirming that only biological processes could account for the identified compounds.
This finding implies that animals emerged much earlier than previously thought. The earliest sponges inhabited ocean floors, filtering water without complex organs, a brain, or a digestive system, yet they had the capacity to metabolize, reproduce, and respond to their environment. Their simplicity and resilience have allowed them to survive into the present day.
Published in the Proceedings of the National Academy of Sciences, this study not only supports the notion that sponges were the first animals but also proposes a methodology for identifying ancient life biomarkers by integrating geology, molecular biology, and organic chemistry. The evidence is based on three converging lines: rock formations, current organisms, and laboratory-replicated chemistry.
These findings could significantly alter the narrative of animal origin, suggesting that complex life may have begun in calm, nutrient-rich seas long before the Cambrian explosion. The team plans to apply this technique in other regions around the world, including continents that once formed a supercontinent, to seek a potential global marker for the origin of animal life. Additionally, this methodology could aid in the search for life on other planets. For instance, if a sterane were discovered on Mars, it could indicate the presence of complex life beyond Earth.
